The invention relates generally to position and/or orientation sensing devices and more particularly to devices for facilitating the tracking of a head or other movable object.
Various devices are known for measuring the orientation of the head (head tracking). Head tracking finds many useful applications. For example, the military uses head tracking to aim gun turrets and to provide heads-up tactical information for the pilot of an aircraft. The private sector uses head tracking as a third hand to allow a handicapped individual to steer a motorized wheelchair, or to allow an individual to manipulate a cursor on a computer screen.
Another application includes the use of head tracking to provide a virtual environment for the user--generally known as "virtual reality". Teachings of such application may be found in U.S. Pat. No. 4,984,179 (Waldern); U.S. Pat. No. 4,884,219 (Waldern); and U.S. Pat. No. 5,130,794 (Ritchey). "Virtual reality describes the host of applications that involve the use of a virtual environment to allow human-machine interaction at a more intuitive level than by typing commands on a keyboard or using a mouse-type device to communicate with a machine. Head tracking is necessary to generate a first-person virtual environment. Typically, the user wears video screens over both eyes and a host computer generates three-dimensional views that correspond to the position of the user's head.
With many known systems, a fixed reference point is compared with the position or orientation of the head by linking the head to the fixed reference point by way of mechanical linkage, optic linkage, ultrasonic linkage, or magnetic linkage, and measuring the position or orientation of the head relative to that fixed reference point. Such systems are described in U.S. Pat. Nos. 4,954,815; 4,565,999; 4,682,159; 5,142,506; and 4,688,037. However, a problem arises with such known head tracking devices since the fixed reference point is off of the body and requires the use of an off-the-body transmitter and receiver configuration that limits the user to a confined space because of the limited range of the transmitters. Known mechanical linkage systems also constrain the user.
The transmitter-receiver configurations typically have a freedom of movement advantage over the mechanical linkage systems but generally require more time to process the received electromagnetic signals before the position/orientation information may be output from the device. This time is referred to as latency or lag time. The off-body mechanical linkage provides almost instantaneous output, or negligible lag time, but severely limits the movement range of the user. Furthermore, such systems tend to be complex and subsequently costly to manufacture and use.
Another device, such as that disclosed in U.S. Pat. No. 5,068,645, offers an advantage over the off-the-body link by using gravity as its fixed reference point and therefore contains only a receiver mounted on the headset. This device is relatively non-restrictive to the user since gravity exists everywhere on the earth. However, such a device may not be readily used in low gravity applications or non-gravity applications such as in outer space. Secondly, this device may be prone to a pendulum effect- The device measures the head position by comparing the orientation of the head to the surface level of a liquid in a spherical container. The liquid's surface stays level due to gravity while the user's head tilts the spherical container. However, the liquid will not stay perfectly level because of inertia, but will rock back and forth when the user's head moves quickly. Such a pendulum effect may hinder the performance of this device. Thirdly, such a device typically only measures the head's angle along a vertical axis. It cannot measure the head's lateral movement along a second axis. Consequently there exists a need for a head tracking device which overcomes the disadvantages of the aforementioned prior art devices.